1) Add MMX deinterlace code.

2) "Fix" first and last line deinterlace.  I had second-thoughts that this might be some image filtering algorithm that someone cleverer than I created.
3) Add in-place deinterlace functions (only used when src == dst).
patch by (Fred <foohoo at shaw dot ca>)

Originally committed as revision 1113 to svn://svn.ffmpeg.org/ffmpeg/trunk